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The RAD18 E3 ligase requires SIVA1 as an accessory protein for binding to its substrate PCNA during translesion DNA synthesis., Translesion DNA synthesis (TLS) is a universal DNA damage tolerance mechanism conserved from yeast to mammals. A key event in the regulation of TLS is the monoubiquitination of proliferating cell nuclear antigen (PCNA). Extensive evidence indicates that the RAD6–RAD18 ubiquitin-conjugating/ligase complex specifically monoubiquitinates PCNA and regulates TLS repair. However, the mechanism by which the RAD6–RAD18 complex is targeted to PCNA has remained elusive. In this study, we used an affinity purification approach to isolate the PCNA-containing complex and have identified SIVA1 as a critical regulator of PCNA monoubiquitination. We show that SIVA1 constitutively interacts with PCNA via a highly conserved PCNA-interacting peptide motif. Knockdown of SIVA1 compromised RAD18-dependent PCNA monoubiquitination and Polη focus formation, leading to elevated ultraviolet sensitivity and mutation. Furthermore, we demonstrate that SIVA1 interacts with RAD18 and serves as a molecular bridge between RAD18 and PCNA, thus targeting the E3 ligase activity of RAD18 onto PCNA. Collectively, our results provide evidence that the RAD18 E3 ligase requires an accessory protein for binding to its substrate PCNA.

Protocols for the stereoselective synthesis of α-(dichloromethyl)amines, α-(chloromethyl)amines, and α-chloroaziridines are presented. Diastereoselective synthesis of α-(dichloromethyl)amines was achieved based on nucleophilic dichloromethylation of aromatic N-tert-butylsulfinyl aldimines with (dichloromethyl)trimethylsilane at a low reaction temperature. Slowly warming the reaction mixture up to room temperature gave α-chloro cis-aziridines. Additionally, with Bu3SnH as the reductant, α-(dichloromethyl)amines were readily obtained from easily accessible α-(trichloromethyl)amines via mono-dechlorination. Over-reduction was successfully suppressed. Subsequent radical mono-dechlorination of the α-(dichloromethyl)amines gave the corresponding α-(chloromethyl)amines in good to excellent yields.